Focused mesh electron multiplier



,Aug. 9, 1966 ML. VESTAL FOCUSED MESH ELECTRON MULTIPLIER Filed Dec. 101963 2 Sheets-Sheet 1 1 I III. :9 e

INVENT OR L. Vesta;

Marz/ b A ORNEYS 9, 1966 M. 1... VESTAL 3,265,916

FOCUSED MESH ELECTRON MULTIPLIER Filed Dec. 10, 1963 2 Sheets-Sheet zATTORNEYS United States Patent M 3,265,916 FOCUSED MESH ELECTRONMULTIPLIER Marvin L. Vestal, Baltimore, Md, assignor to William H.

Johnston Laboratories, Inc, Baltimore, Md., :1 corporation of MarylandFiled Dec. 10, 1963, Ser. No. 329,540 7 Claims. (Cl. 313-405) Thisapplication relates to electron multiplication, and more particularly,to an electron multiplier in which a focusing effect is achieved with agrid type of structure.

Electron multipliers are in general use today for a number of differentpurposes. For instance, such multipliers are employed to furnish a highintensity source of electrons, are employed to change a photographicimage into an electron image, with appropriate multiplication oramplification thereof, and are further employed in conjunction withcounting of ions, electrons, and other forms of radiation in, forinstance, mass spectroscopy.

Electron multipliers currently available on the market may be dividedinto three fundamentally different types. The first type employsmagnetic focusing, While the second type employs pure electrostaticfocusing. The third type available in the type mostnearly similar tothat of the present invention and employs meshes or grid-s which arepositioned in stacked array between the source of radiation (whetherelectrons or ions) and the anode from which the amplified stream ofelectrons is extracted. An alternative form of the grid or mesh type ofmultiplier is one identified as the E.'M.I.-type and uses grids whichare formed in the design of venetian blind slats.

The grid or mesh type of multiplier is fundamentally simpler and morecompact than the commercially-available electrostatic and magneticmultipliers, but the types generally available on the market today areinherently relatively inefiicient. In fact, the commercially-availablemagnetic and focused electrostatic multipliers are generally ofconsiderably higher gain and higher collection efficiencies than theunfocused electrostatic multipliers which use a mesh or grid dynodeconfiguration.

It is an object of this invention to provide an electron multiplierwhich is relatively compact and simple in comparison with thecommercially-available magnetic and focused electrostatic multipliers,but which also has the high gain and collection efficiencies normallyassociated with these types of multipliers. It is a further object ofthis invention to provide a simple and compact construction of electronmultipliers, with high gain and high collection efficiency.

These and other objects of the invention are achieved by use of thestacked grid type of construction normal-1y associated in the commercialmarket with the unfocused electrostatic multiplier, but to employ suchgrids, of unique con-figuration, in conjunction with focusing means suchas to enhance the gain collection efficiency of the multiplier.

The prior art does contain a suggestion of the use of focusing inconjunction with grid or mesh-type multipliers, in British Patent No.516,6211 In this patent a series of stacked grids containingpassage-defining depressions of diminishing cross section in thedirection of electron travel are disclosed as provided with annularflanges which are successively biased to a higher positive voltage, toprovide a degree of focusing. An improvement over the disclosure of thisBritish patent is disclosed in British Patent No. 543,106 in which thegrids are disclosed as provided with depressions in hexagonal array,such depressions merging with each other adjacent the entrance side ofeach dynode grid, and terminating at the opposite side in circularholes. Thereby, a number of ridges are provided which are exposed toprimary electrons through the holes in preceding dynode grids.Alternatively, the later British patent sug- Patented August 9, 1966gests the use of spherical, rather than polygonal depressionss, with ahexagonal array of ridges between each set of six depressions.

The present invention, as distinguished from the subject matterdisclosed in the above British patents, employs meshes or grids asdynodes, but with material removed from the dynode plates or sheets insuch fashion as to form a single cusp defined by each set of fouradjacent holes, with the holes and the depressions extending outwardlytherefrom arranged in square, as distinguished from hexagonal, array.Further, the present invention employs focusing plates associated witheach one of the dynode plates or grids, with such focusing platescontaining cylindrical passages therethrough which are arranged oppositethe users and therefore expose each cusp of the dynode plate or grid tothe primary electrons emanating either from the source or from theprevious stage of the electron multiplier. The combination of the dynodeand the focusing plate is biased to the same voltage, by being inmechanical contact together, thereby providing for materially-enhancedcollection efiiciency and greater gain.

As will be indicated more fully hereinafter, the electron multiplier ofthe present invention employs a number of stages, each one of whichconsists of a dynode plate or grid of the indicated configuration, and afocusing plate constructed and aligned as described, each stage ofmultiplication being staggered or displaced in respect to thedepressions and holes, as compared to the previous stage, so thatprimary electrons which strike the cusp of one dynode release secondaryelectrons which are then directed through the passage of the succeedingfocusing plate onto the cusp of the next dynode, with no straight pathfor primary electrons through the various stages. With the configurationof depressions and holes in the dynodes and focusing plates provided bythe present invention, it is possible to manufacture all of theseelements in the same identical manner, for a complete electronmultiplier of many stages, and to provide for the staggered arrangementby mere rotation of successive stages with respect to the previousstage.

The apparatus of the invention will now be more fully described inconjunction with a preferred embodiment thereof shown in theaccompanying drawings.

In the drawings,

.FIG. 1 is an exploded perspective view showing the focusing plate,dynode and insulating ring of one multiplier stage;

. FIG. 2 is a plan view of an electron multiplier of the invention, withthe guard or focusing plate removed; and

FIGURE 3 is a sectional view taken along line 3-3 of FIGURE 2 but with afocusing plate in position-in the first stage of the multiplier.

Referring first to FIG. 1, each multiplying stage of the inventionincludes as an essential part thereof, a dynode plate 10 which is shownas being of disc-shape with a number of passages 11 indicated asextending therethrough. The passages will be more fully describedhereinafter. Each dynode plate is provided with mounting and positioningholes 12 which extend from one opposite face of the dynode plate to theother.

In addition to the dynode plate 10, at least each stage except the firststage of the electron multiplier is provided with a focusing plate 13which has mounting holes 14 extending therethrough and mating withcorresponding holes 12 in the dynode 10'. The focusing plate 13 is alsopro- .vided with passages 15, of cylindrical nature, extending betweenopposite sides thereof, and the plate, like the dynode plate 10, is ofdisc-shape, so as to be cylindrical in outside configuration.

The several stages of the electron multiplier of the invention must beelectrically isolated from each other, or

insulated from each other, and this function may be provided by spacingrings 16 of insulating material. Such rings have mounting holes 17extending therethrough which register with the corresponding holes 12and 14 in the dynode and the focusing plate 14, respectively. The rings16 are hollow, as shown so that the active surfaces of the dynode andthe focusing plate through which the holes and passages extend are notcovered by the insulating rings.

Referring now to FIGS. 2 and 3, the structure of the various elements ofthe multiplier will be described in more detail. As indicated above,FIG. 2 is a plan view of the electron multiplier shown in cross sectionin FIG. 3, but with the first stage focusing plate 13 removed. As amatter of fact, it is not at all essential that a focusing plate beprovided for the first stage, and in one embodiment of the invention aguard plate consisting merely of a metal disc with a central cylindricalpassage therethrough of dimension corresponding to the entire activesurface of the dynode 10, was provided, this passage or hole beingcovered by a conventional wire mesh of about 50 lines per inch. Thisguard plate then performed the function of establishing a reference forthe necessary electrostatic fields of the apparatus, but allowed theelectrons or positive ions directed toward the first stage substantiallyfree and unimpeded access to all areas of the first dynode 10.

Alternatively to the structure described immediately above, the electronmultiplier could also be provided with a guard plate of characteristicsdescribed above positioned in front of the first focusing plate 13, butspaced and insulated therefrom, and this guard plate could form theelectrical field termination point for the electrostatic focusing fieldsof the invention.

Each of the dynode plates of the apparatus of the invention is of anyappropriate electron-emissive material such as to have thecharacteristic of emitting a larger number of secondary electrons thanthe number of primary electrons or positive ions which strike it. Such amaterial is a beryllium surface suitably oxidized in well known manner,but this invention is not to be considered limited to any particularkind of secondary electron-emissive material forming the surface of thedynode plate 10. In fact, in one actual construction of the invention,the dynode plate 10 was of a silver-magnesium alloy, with a magnesiumoxide coating activated in manner well known to the art.

As indicated above, the dynode plate 10 is provided with a large numberof passages 11 extending therethrough between opposite faces of :theplate, in the axial direction. These passages 11 extend symmetricallyoutwardly from central holes 20, in the rear face of the plate, incraterlike fashion, to the opposite or front surface of the plate. Theactual configuration of the passages 11 may be more readily understoodby a description of a suitable manner or method of making them.

It is desired to leave between each set of four passages 11 through thedynode plate 10, a single cusp 21. This cusp is defined by sloping wallscorresponding to intersecting depressions of crater-like form, whichdepressions terminate in the holes 21. The passages 11 are convenientlymade by masking all of the exposed surfaces of the dynode plate 10, withthe exception of areas corresponding to the holes at the rear sides ofthe plates. The spacing of these holes will be described hereinafter,but it will be evident from FIG. 2, that the holes are uniformly spacedalong the vertical and horizontal directions of that figure. The maskingis to protect the plate against etching by a suitable chemical etchant,such as the acids, or acidic materials, ordinarily employed in theprinting and printed circuit industries, and may be any of the resisttype of materials known depending upon the etchant selected and themetal employed for the dynode plate 10.

When the dynode plate 10, suitably masked or protected by resistmaterial in the manner indicated, is immersed in an etchant ofappropriate nature, or an etchant is sprayed thereagainst, the areas ofthe plate attacked initially are only those exposed by the holescorresponding to the holes 20, in the resist material. Since removal ofmetal proceeds with equal speed in all directions from the initial hole20, the spaces left by the etchant will broaden out from the initialholes 20 until the plate is completely etched through. When this resultis obtained, the plates are immediately removed from the etchant, andthe resistant material may be removed from the plates.

The passages 11 on the front side of the dynode plate will thenterminate in surfaces which are curvilinear in nature, with only cusps21 left between each four of the passages 11. These cusps will belocated at a radius (R) from the center of the hole 20 which is equal tothe sum of the radius of the hole 20 and 'the thickness (t) of the plate10. If the holes were not of the proper distance apart, but were ratherof a larger distance, rather than a cusp 21 the etching process wouldleave a circular ridge surrounding each passage, with the extent of theridge between adjacent passages being determined by the distance betweenpassages. However, in the present invention it is essential that only asingle cusp be left between each set of four passages, and this resultis achieved by choosing the distance (d) between the centers of holes 20equal to the square root of 2 multiplied by the sum of the radius (r) ofholes 20 and the thickness (t) of the plate 10. If this cusp is to beobtained, the radius of hole 20 must be less than the square root of 2divided by 2, multiplied by the thickness (t), for otherwise no cuspwould be left but rather all of the material dividing adjacent holeswould be removed. A practical condition for the retention of the cusps21 is that the radius (r) of the hole 20 be equal approximately to halfthe thickness (2).

Another extremely important characteristic of the placement of thepassages 11 in the dynode plates 10 is that the rows of the holes in thedynode plates next adjacent perpendicular diameters of the plates beoffset from those diameters by substantially one-fourth of the distancebetween adjacent holes. In other words, referring particularly to FIG.2, as is indicated by the section line 3-3, the centers of the middlehorizontal row of holes 20 are spaced downwardly with respect to thediameter passing through the mounting holes 12, by the distanceindicated. Also, the centers of the middle vertical row of holes 29 aredisplaced from the vertical diameter passing through the upper and lowermounting holes 12 by the same distance. With :this arrangement, thedynodes may be all manufactured identically, with the holes and passagesall identically located, yet the dynodes of successive stages may beproperly positioned by mere rotation with respect to each other, so:that the hole 20 in one dynode is opposite the cusp 21 in the nextdynode. This relationship is shown in FIG. 3, wherein the second stagedynode plate 10' has its cusps 21' opposite the holes 20 in the firstdynode plate 10.

The focusing plates 13 may be manufactured in similar manner as thedynode plates 10, but they are merely provided with cylindrical passages15 which are identically located with respect to the holes 20 in thedynode plates. Then, in each electron multiplication stage, the focusingplate 13 may be positioned such that its passages 15 are opposite thecusps 21 of the associated dynode plate 10, and in the next stage thepassages 15' will then be opposite the holes 20 in the preceding dynodeplate. The relative placement of holes and cusps is shown by the dottedoutline of these elements in FIG. 2, as well as FIG. 3.

While one very advantageous way of making the passages 11 through thedynode plates and the passages 15 through the focusing plates has beendescribed above, it will be evident that methods of mechanical removalmight be devised to perform substantially the same function.Consequently, the invention is not to be considered limited, exceptwhere required by the claims, to this particular method of manufactureof these plates, but it will be seen that the surfaces of the dynodeplates are specifically most conveniently described by reference to themethod of removal employed.

Referring to FIG. 3, it will be seen that the electron multipliertherein shown comprises a series of stages of multiplication, each ofwhich includes a dynode plate 10. These stages are assembled together,with the insulating spacing rings 16 between them and clamped inposition, with a solid anode plate 25 at the end of the stack ofmultiplication stages remote from the entrance of the electrons or ionsto which the multiplier is to respond. In other words, the anode plateis adjacent the last stage of the electron multiplier. This anode plate25 may be provided with mounting holes 26 corresponding to the mountingholes 14, 12 and 17 in the focusing plate, dynode and insulating ring,respectively.

When clamped together, with the aid of mounting rods and clamps, (notshown) the dynode plates and focusing plates 13 will be seen to be indirect physical contact, so that any potential applied to one of themwill also be applied to the other. Each stage of multiplication is thenspaced apart and insulated from each other.

In order to bias the dynode plates and focusing plates appropriately foracceleration of the secondary electrons emitted from one dynode towardthe next stage of multiplication, a suitable source of direct currentvoltage, such as indicated by the battery 27, may be provided. Thevoltage from this source may then be subdivided by the usual voltagedivider consisting of a series network of resistors, (not shown) toprovide discrete voltages which may be supplied to successive stages ofthe multiplier. Alternatively, and as shown in FIG. 3, the insulatingrings 16 may be selected to be of appropriate resistivity to providevoltage drops therethrough so that, with proper selection of the voltagesource 27, each stage of electron multiplication is biased to asuccessively higher positive voltage with respect to the precedingstage.

In operation of the electron multiplier of the invention, the multiplierwill either be assembled in its own evacuated envelope (not shown); orit may be positioned in an evacuated container such as employed for massspectroscopy invest gations. The electrons or ions which are to beamplified may then be directed at the upper surface of the electronmultiplier, as shown in FIG. 3, and as indicated by the arrows in thatfigure. If a guard plate of the type described above is employed ratherthan the first focusing plate 13, the mesh will cover the entire activearea of the first dynode plate 10 and will permit substantially all ofthe electrons or ions which reach it to pass therethrough and intocontact with the surfaces of the passages 11. If the first focusingplate 13 is used, however, only those electrons or ions which passthrough the holes 15 will strike the first dynode plate 11.

The dynode plate 10 will be struck by the energizing stream of electronsor ions in the areas immediately surrounding the cusps 21, and will emitsecondary electrons of number greater than the number of particlesstriking it. In other words, for each primary electron or ion strikingthe region immediately around the cusp 21, a

plurality of secondary electrons will be emilted. In the absence of thefocusing plates 13, the biases provided by the dynode plates 10 and anyguard plate or other device defining a ground plane for the multiplier,would urge such secondary electrons to return to the surfaces from whichthey were emitted. However, with the focusing plates 13 at the samepotentials as the dynode plates 10, the only electric fields operativeupon such secondary electrons are those between each dynode plate andthe focusing plate of the successive multiplier stage. These fields,which in effect penetrate the holes in the dynode plates 10, tend toaccelerate the secondary electrons toward the holes or passages 15 inthe focusing plate of the next mutliplier stage. As such electrons passthrough the passages 15, they strike the surfaces immediately adjacentthe cusps 21 of the next dynode plate and cause such surfaces to emitsecondary electrons, in the same manner as did the preceding dynodeplate. In this manner, multiplication occurs and the secondary electronsare finally directed to the anode 25 Where they are collected forsubsequent electronic tube or transistor amplification, or forindication.

It will be understood that the representation of the electron multiplierof the invention furnished by the drawings of this application does notnecessarily correspond in size to the actual practical commercialembodiment of the multiplier. In fact, such sizes have been distortedfor ease of representation, but it will be evident that the number ofpassages 11 in the dynodes and the corresponding number of passages 15in the focusing plates would be much larger than shown.

An illustrative embodiment of an electron multiplier of the inventionemploys dynode plates which are 0.02 inch thick, with focusing plateswhich are substantially thinner, and in fact are about 0.004 inch thick.The anode plate, as well as the guard plate, if one is employed, are0.02 inch thick, like the dynode plates. The dynode plates and focusingplates, as well as any guard plate employed, are provided with an activecentral area of 1.5 inches diameter. Within this central area arelocated the passages 15 in the focusing plates and the passages 11 inthe dynode plates. There may be 36 holes along the central horizontalline, and 36 holes along the vertical central line, such lines beingdisplaced from the corresponding diameters of the dynode plates by0.0106 inch. The center of each .hole is then displaced from itsneighbor in a horizontal and a vertical direction by equal spaces of0.0424 inch, and each hole is of 0.020 inch diameter.

The number of stages to be assembled together is not at all criticalwith the invention, and as indicated in FIG. 3, a large number of stagescan be assembled and joined together. However, the cusps 21 wouldnormally be quite sharp if made by the etching method recommended, and,to avoid noise caused by field emission from sharp edges, the cuspsshould be rounded off to about 0.002 minimum radius after etching.

If the standard type of voltage divider is to be employed with theelectron multiplier of the invention, the dynode plates may beappropriately provided each with one or more ears extending outwardlyfrom its outer surface, to which electrical connection may be made toprovide the stages with the necessary successively higher acceleratingvoltages. Alternatively the focusing plates could be provided with suchears and connections made thereto.

It will be evident that it is not essential to the present inventionthat the dynodes and focusing plates be cylindrical in configuration. Infact other configurations are completely feasible. However, it isessential to the present invention that the passages in the dynodeplates, and the corresponding passages in the focusing plates, be ofsquare array, with each set of four of the passages in the dynode platesdefining a single cusp between them, for the purposes indicated above.

It will be apparent that many minor changes could be made in theapparatus specifically illustrated and de scribed above. The inventiontherefore is not to be considered limited to this specific embodiment,but rather only by the scope of the appended claims.

I claim:

1. A plural stage electron multiplier for supplying an amplified outputcorresponding to an input from a stream of electrons or ions, and inwhich each stage comprises:

a dynode plate of electron-emissive material having the property ofemitting a number of secondary electrons higher than the number ofprimary electrons or ions which strike it, said plate having surfacesdefining a plurality of passages extending therethrough from oneopposite face to the other, in square array, the surfaces defining eachpassage sloping outwardly in crater-like fashion from a central hole ofradius r in one of said faces of the dynode plate to the opposite facein such fashion that the surfaces of each set of four adjacent ones ofsaid passages define a cusp at radius R from the center of each one ofsaid four holes, where R is substantially equal to r+t, t being thethickness of said plate between said opposite faces, and, in at leasteach stage but the first stage in the direction of the stream of theelectrons or ion-s, a focusing metal plate of thickness much smallerthan that of said dynode plate having surfaces defining cylindricalpassages substantially of radius r extending between opposite facesthereof, the passages of said focusing plate being of the same numberand geometrical arrangement .as the cusps in the dynode plate, saidfocusing plate having one of its opposite faces in metallic contact withthe face of the dynode plate bearing said cusps, with the passages inthe focusing plate opposite the cusps, so that primary electrons passingthrough the passages in the focusing plate will strike the surfaces ofthe dynode plate adjacent said cusps to cause emission of secondaryelectrons therefrom; said stages being spaced apart along the directionof the passages through the focusing plates, with each stage insulatedfrom the others, and with the focusing plate of every stage except thefirst next adjacent the dynode plate of the previous stage, with thepassages through the focusing plate opposite the said holes in thedynode plate, means for biasing successive stages to successively higherpositive voltages with respect to the first stage, and means forcollecting electrons from the last stage. 2. The apparatus of claim 1 inwhich each of said cusps is rounded 01f to inhibit field emission ofelectrons. 3. The apparatus of claim 2 in which each of said dynode andfocusing plates is cylindrical in shape with the said opposite facesthereof being spaced apart axially thereof and with the centers of theholes in the dynode plates and the passages in the focus-ing plateswhich are next adjacent the perpendicular diameters of the plates beingoffset from those diameters by substantially onefourth the distancebetween the centers of adjacent holes.

4. The apparatus of claim 3 in which the distance d between each set ofadjacent holes in each dynode plate is substantially equal to the squareroot of 2 multiplied by the sum of the radius r of that hole and thethickness t of the dynode plate.

5. The apparatus of claim 4 in which the surfaces defining the passagesthrough the dynode plates are shaped correspondingly to the surfaceswhich would result if each dynode plate had all of its surfaces maskedagainst an etchant for the plate except for the areas of said holes inthe dynode plates, and the dynode plates were immersed in such etchantfor a time sufficient for etching to proceed through the plate from theface bearing said holes to the opposite face.

6. The apparatus of claim 5 in which said electroncollecting means is acylindrical anode plate aligned with but insulated from the dynode ofthe last stage of the multiplier, to receive and collect secondaryelectrons emitted therefrom.

7. The apparatus of claim 6 in which said biasing means includes ringshaped insulators in contact with and spacing apart the rims of themetal plates of adjacent stages and the anode plate, said insulatorsbeing predetermined resistivity,

and a direct current voltage source connected between the first stageand the anode plate, said source supplying a voltage of magnitude incomparison with the resistivity of said insulators such as toappropriately bias each stage in accordance with the voltage drop acrosseach insulator.

No references cited.

JAMES W. LAWRENCE, Primary Examiner.

R. SEGAL, Assistant Examiner.

1. A PLURAL STATE ELECTRON MULTIPLIER FOR SUPPLYING AN AMPLIFIED OUTPUTCORRESPONDING TO AN INPUT FROM A STREAM OF ELECTRONS OR IONS, AND INWHICH EACH STAGE COMPRISES: A DYNODE PLATE OF ELECTRON-EMISSIVE MATERIALHAVING THE PROPERTY OF EMITTING A NUMBER OF SECONDARY ELECTRONS HIGHERTHAN THE NUMBER OF PRIMARY ELECTRONS OR IONS WHICH STRIKE IT, SAID PLATEHAVING SURFACES DEFINING A PLURALITY OF PASSAGES EXTENDING THERETHROUGHFROM ONE OPPOSITE FACE TO THE OTHER, IN SQUARE ARRAY, THE SURFACESDEFINING EACH PASSAGE SLOPING OUTWARDLY IN CRATER-LIKE FASHION FROM ACENTRAL HOLE OF RADIUS R IN ONE OF SAID FACES OF TE DYNODE PLATE TO THEOPPOSITE FACE IN SUCH FASHION THAT THE SURFACES OF EACH SET OF FOURADJACENT ONES OF SAID PASSAGES DEFINE A CUSP AT RADIUS R FROM THE CENTEROF EACH ONE OF SAID FOUR HOLES, WHERE R IS SUBSTANTIALLY EQUAL TO R+T, TBEING THE THICKNESS OF SAID PLATE BETWEEN SAID OPPOSITE FACES, AND, INAT LEAST EACH STAGE BUT THE FIRST STAGE IN THE DIRECTION OF THE STREAMOF THE ELECTRONS OR IONS, A FOCUSING METAL PLATE OF THICKNESS MUCHSMALLER THAN THAT OF SAID DYNODE PLATE HAVING SURFACES DEFININGCYLINDRICAL PASSAGES SUBSTANTIALLY OF RADIUS R EXTENDING BETWEENOPPOSITE FACES THEREOF, THE PASSAGES OF SAID FOCUSING PLATE BEING OF THESAME NUMBER AND GEOMETRICAL ARRANGEMENT AS THE CUSPS IN THE DYNODEPLATE, SAID FOCUSING PLATE HAVING ONE OF ITS OPPOSITE FACES IN METALLICCONTACT WITH THE FACE OF THE DYNODE PLATE BEARING SAID CUSPS, WITH THEPASSAGES IN THE